1. Field of the Invention
Embodiments of the present invention relate to a vehicle control apparatus configured to control a vehicle equipped with an electric motor.
2. Description of the Related Art
One example of a control method for driving a three-phase alternating current (AC) motor is pulse width modulation (PWM) control. In the PWM control, a power converter is controlled in accordance with a magnitude relation between phase voltage command signals, which are set from a viewpoint of matching phase current supplied to the three-phase AC motor with a predetermined value, and a carrier signal of a predetermined frequency.
When the three-phase AC motor is driven, a ripple in direct current (DC) voltage inputted to the power converter (i.e. a so-called pulsating component) increases in some cases. For a method of reducing the ripple, for example, in Japanese Patent Application Laid Open No. 2009-171768, there is proposed a technique/technology in which a third harmonic signal is superimposed on the phase voltage command signals and in which correction is performed to reduce an amplitude shift and a phase shift of the third harmonic signal.
On the other hand, when the three-phase AC motor is driven and has an extremely low number of revolutions, heating of switching elements of the power converter increases in some cases, which may cause failure. For a method of reducing the heating of the switching elements, for example, in Japanese Patent Application Laid Open No. 2016-054594, there is proposed a technique/technology in which a center value of the phase voltage command signals is offset-corrected to reduce losses of the switching elements.
The present inventors' studies show that the ripple generated on the DC voltage side of the power converter increases if the correction described in Japanese Patent Application Laid Open No. 2016-054594 (i.e. the offset of the center value of the phase voltage command signals) is performed. This is because shifting of the phase voltage command signals causes an extension in a zero vector period in which all the switching elements in three phases are off.
In order to reduce the ripple that has increased, it is also possible to adopt the correction described in Japanese Patent Application Laid Open No. 2009-171768 (i.e. the correction for the amplitude shift and the phase shift of the third harmonic signal); however, a sufficient correction effect cannot be obtained while the center value of the signals is offset, and the ripple cannot be reduced. In other words, the ripple that has increased due to the offset of the center value of the signals cannot be reduced by the correction for the amplitude shift and the phase shift of the third harmonic signal.
As a result, if the ripple in the DC voltage increases due to the offset of the center value of the phase voltage command signals, it is hard to reduce the ripple, resulting in malfunction, such as, for example, overpressurization and unstable motor control, which is technically problematic.